How do I enable file preview for files stored on a NAS in Windows?

Applicable Products

• All QNAP NAS series

Overview

When accessing files such as PDF or Microsoft Office documents stored on a QNAP NAS from Windows File Explorer, the Preview pane may not display the file content. Instead, Windows shows a warning message:

“This file might be unsafe to preview. If you trust the file and its source, open it to view its contents.”

This behavior is not caused by a QNAP NAS issue. Starting from Windows security updates released after October 2025 (including Windows 25H2), Microsoft introduced stricter security controls to mitigate vulnerabilities such as NTLM hash leakage. As a result, Windows may block the preview of certain file types from network sources.

Windows blocks file preview in the following scenarios:

• Files with Mark of the Web (MOTW): Files downloaded from browsers, email, or instant messaging apps may have MOTW metadata, causing Windows to block preview.
• Files accessed via a network (UNC path): Files accessed through UNC paths (e.g., \\NASName\Share\file.pdf) are treated as coming from the Internet Zone, resulting in preview restrictions.
• Files in the Downloads folder: Files stored in the Windows Downloads folder may retain MOTW and be blocked from preview unless unblocked.

Solution

Method 1: Add the NAS to the Local Intranet zone (Recommended)

1. Press Win + R, enter inetcpl.cpl, and press Enter.
2. Go to the Security tab.
3. Select Local intranet, then click Sites.
4. Click Advanced.
5. Add your NAS address, for example:
   • NASName
   • 192.168.1.10
   • 192.168.1.*
6. Click OK to apply the settings.

After this configuration, Windows will treat the NAS as a trusted source and allow file preview.

Method 2: Map the NAS share as a network drive

1. Right-click This PC in File Explorer.
2. Select Map network drive.
3. Enter the NAS share path (e.g., \\NAS\ShareName).
4. Assign a drive letter (e.g., T:\ or Z:\).
5. Click Finish.

Accessing files through the mapped drive enables file preview in most cases.

Method 3: Remove the Mark of the Web (MOTW) from files (If applicable)

• Unblock a single file:
  1. Right-click the file and select Properties.
  2. Check Unblock and click Apply.
• Batch unblock using PowerShell:

  Get-ChildItem -Path "T:\Files" -Recurse | Unblock-File

Recommended actions:

• Add the NAS to the Local Intranet zone.
• Access files using a mapped network drive instead of a UNC path.

These steps allow file preview to function correctly while maintaining system security. This is a Windows client-side issue related to NTLM hash protection, Mark of the Web (MOTW), and Windows trust zones, not a malfunction of the NAS.